1. Field of the Invention
The present invention relates to a commutator motor comprising a commutator having a hollow cylindrical insulating resin portion and a plurality of segments on an outer circumferential portion of the insulating resin portion with the spaces between each being held by the insulating resin portion.
2. Description of the Related Art
FIG. 16 is a cross-section of a conventional commutator motor 100. The commutator motor 100 comprises: a cylindrical yoke 101; permanent field magnets 102 secured inside the yoke 101; a shaft 104 disposed inside the yoke 101 by means of bearings 103 so as to be able to rotate freely; an armature 105 secured to the shaft 104; a commutator 106 secured to an end portion of the shaft 104; and brushes 108 held by brush holders 107 contacting the surface of the commutator 106 by the elastic force of springs (not shown).
The armature 105 comprises: a core 109 having a plurality of slots 110 extending longitudinally; and windings 111 constructed by winding wire into the slots 110 by a lap winding method.
The above hollow, cylindrical commutator 106 is provided with a plurality of copper segments 112 arranged with even pitch, and mica (not shown) disposed between the segments 112 for electrically insulating adjacent segments 112.
In the above 4-pole lap-wound commutator motor 100, an electric current is supplied to the windings 111 from outside by means of the brushes 108 contacting the segments 112, whereby the armature 105 rotates together with the shaft 104 due to electromagnetic action.
In a commutator motor 100 of the first construction above, mica is interposed between the segments 112, forcing the commutator motor 100 to be enlarged. For that reason, there are commutator motors aiming to reduce size by eliminating the mica.
FIG. 17 is a front cross-section of a commutator motor 113 of a commutator motor of a second construction. As shown in FIG. 18, the commutator 113 is formed by integrating an insulating resin portion 114 and an annular portion 115 composed of copper by insertion molding, then cutting a plurality of spaces 116 longitudinally in the outer surface of the annular portion 115. In other words, the commutator 113 is formed such that spaces 116 are formed between the segments 117 and the segments 117 are arranged with even pitch circumferentially.
In a commutator motor of the second construction above, all of the segments 117 of the commutator 113 are held by the insulating resin portion 114, but when the cutting precision is insufficient, irregularities occur in the width W of the segments 117 and in the dimensions L of the spaces between the segments 117. Then, as shown in FIG. 19, for example, where three segments 117 should normally contact the first brush 108A and the third brush 108C and two segments 117 should normally contact the second brush 108B and the fourth brush 108D, for just an instant only two segments 117a, 117x (the first and twenty-second) contact the first brush 108A. As a result, where the number of coils 118 in this case should normally be evenly distributed at four for each of the first to fourth circuits 119a to 119d between the first to fourth brushes 108A to 108D, a different number arises. That is to say, the first coil 118a to the fifth coil 118e are interposed in the first circuit 119a, making five interposed coils 118. Consequently, where the current A flowing through the first circuit 119a should normally have the same value as the current B flowing through the third circuit 119c, a different value arises. As a result, a problem has been that the balance of electromagnetic attraction acting on the armature 105 is lost, giving rise to excitation in the armature 105 and generating operating noise.
Besides requiring highly precise cutting of the annular portion 115 in order to avoid problems of deviation in the commutation timing resulting from the above sort of irregularities in cutting precision, as shown in FIG. 20, in order to prevent the brushes 108 from shifting around the circumference of the commutator 113, the clearance between the brushes 108 and the brush holders 107 is made as small as possible, and for that reason friction arises between the brushes 108 and the brush holders 107, and so a problem has been that in order to overcome that frictional force and place the brushes 108 in contact with the segments 117, the force pressing against the brushes 108 must be large, increasing torque loss.